-Many people advocate doing RtK before you start learning grammar and vocabulary. I think that this is a bad idea for most people because it takes too long and it's easy to lose motivation. It's too long before you can even say "Where's the bathroom?"

It's a bit of an investment. No, you're not actually learning to speak Japanese while going through RTK, but it makes the ride after it a whole lot easier. Think of how many times you see people complaining about how difficult kanji are. I'd wager much more people have lost motivation due to frustration with kanji along the way.

It's also not necessarily true that you can't learn anything else while you're going through the book. If you're tired of studying kanji, read up on the language, grammar, watch some videos, whatever. Then come back later when you feel reinspired. One of the best feelings of accomplishment I felt was when I reached a kanji in RTK that I'd come across somewhere else before.

-By design, the kanji aren't ordered by how useful/common they are. I think there's no sense in learning the less common kanji before you get to an intermediate level; you'll rarely see them in use.

The point is that you go through all the common use kanji in fell swoop. Get it out of the way. If that's the goal, then it makes no sense to order them by frequency, because by the end you will have had to study all of them anyway. Again, it's a long-term investment.

-RtK focuses on writing, which I do think is not as important for everybody, depending on whether they want to live/work in Japan.

Whether writing is important or not is debatable, but I'll give you that. There's a different reason RTK will have you write kanji though, which is that it helps you familiarize with them (the whole point of the book). It helps you go from "mess of squiggles" to an actual character with individual strokes.

-RtK focuses on a link between the kanji and an English word for the meaning, which I don't ultimately think is that useful. There are a lot of cases where the meaning of a compound isn't obvious from the constituent kanji; there are a lot of cases where a kanji has multiple meanings because of idiomatic or historical usage. When you see the word 手紙 you need to know that it means "letter" and it's pronounced てがみ. You don't need to know that it's made up of hand + paper, and if you don't know the word, "hand + paper" is not really going to help you guess. And there are kanji like 温 and 暖, which both mean something like "warm" but have different contexts in which they're used; you don't achieve anything meaningful by finding different English keywords for them.

The keywords are just that, keywords. The memory is made of associations, so it helps a lot when you're able to associate a kanji with something you already know. When you become more familiar with the kanji due to vocab study and such, you forget the keywords. (This is also why you're not supposed to go kanji -> keyword.)